Quantum geometric photocurrents of quasiparticles in superconductors

TL;DR

This paper develops a theory linking nonlinear optical photocurrents in superconductors to the quantum geometry of Bogoliubov-de-Gennes quasiparticles, enabling contactless probing of superconducting properties and symmetries.

Contribution

It introduces a comprehensive theory connecting photocurrents to the quantum geometric tensor of BdG quasiparticles in superconductors, including symmetry considerations.

Findings

Photocurrents depend on the quantum metric and Berry curvature dipole.

Photocurrents can probe time-reversal symmetry breaking.

Application to twisted bilayer and multilayer superconductors.

Abstract

Nonlinear optical response is a sensitive probe of the geometry and symmetry of electronic Bloch states in solids. Here, we extend this notion to the Bogoliubiov-de-Gennes (BdG) quasiparticles in superconductors. We present a theory of photocurrents in superconductors and show that they sensitively depend on the quantum geometry of the BdG excitation spectrum. For all light polarizations, the photocurrent is proportional to the quantum geometric tensor: for linear polarized light it is related to the quantum metric and for circular polarization -- the Berry curvature dipole of the associated BdG bands. We further relate the photocurrent to the ground state symmetries, providing a symmetry dictionary for the allowed photocurrent responses. For light not at normal incidence to the sample, photocurrent probes time-reversal symmetry breaking in systems with chiral point groups (such as twisted bilayers). We demonstrate that photocurrents allow to probe topology and TRS breaking in twisted $d$-wave superconductors and test the nature of superconductivity in twisted WSe$_2$ and multilayer stacks of rhombohedral graphene. Our results pave the way to contactless measurement of the quantum geometric properties and symmetry of superconductivity in materials and heterostructures.